This commit adds a derivation `gcc-stageCompare` to
`pkgs/test/stdenv/default.nix`.
It is important to always build this derivation whenever building
`stdenv`! Because we are using a Nix-driven bootstrap instead of
gcc's built-in `--enable-bootstrap`, the `gcc` derivation no longer
performs the post-self-compilation sanity check. You must build
this derivation in order to perform that sanity check.
The major benefit of this new approach is that the sanity check
(which involves a third compilation of gcc) can be performed
*concurrently* with all packages that depend on `stdenv`, rather
than serially. Since `stdenv` has very little derivation-level
parallelism it cannot take advantage of more than one or perhaps two
builders. If you have three or more builders this commit will
reduce the time-to-rebuild-stdenv by around 20% (one of three gcc
rebuilds is removed from the critical path, and stdenv's build time
is dominated by roughly 3*gcc + 1*binutils + 1*bison-test-suite).
Co-authored-by: Sandro <sandro.jaeckel@gmail.com>
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